1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to an apparatus and method for transmitting/receiving a pilot sequence in a mobile communication system using a space-time trellis code (hereinafter referred to as “STTC”).
2. Description of the Related Art
With the rapid development of mobile communication systems, the amount of data serviced by the mobile communication system has also increased. Recently, a 3rd generation mobile communication system for transmitting high-speed data has been developed. For the 3rd generation mobile communication system, Europe adopts an asynchronous wideband-code division multiple access (hereinafter referred to as “W-CDMA”) system as its radio access standard, while North America adopts a synchronous code division multiple access-2000 (hereinafter referred to as “CDMA-2000”) system as its radio access standard. Generally, in these mobile communication systems, a plurality of mobile stations (MSs) communicate with each other via a common base station (BS). However, during high-speed data transmission in the mobile communication system, a phase of a received signal may be distorted due to a fading phenomenon occurring on a radio channel. The fading reduces amplitude of a received signal by several dB to several tens of dB. If a phase of a received signal distorted due to the fading phenomenon is not compensated for during data demodulation, the phase distortion becomes a cause of information error of transmission data transmitted by a transmission side, causing a reduction in the quality of a communication service. Therefore, in order to transmit high-speed data without a decrease in the service quality, mobile communication systems must overcome fading, and use several diversity techniques in order to do so.
Generally, a CDMA system adopts a rake receiver that performs diversity reception by using delay spread of a channel. While the rake receiver applies reception diversity for receiving a multipath signal, a rake receiver applying the diversity technique using the delay spread is disadvantageous in that it does not operate when the delay spread is less than a preset value. In addition, a time diversity technique using interleaving and coding is used in a Doppler spread channel. However, the time diversity technique is disadvantageous in that it can hardly be used in a low-speed Doppler spread channel.
Therefore, in order to cope with fading, a space diversity technique is used in a channel with low delay spread, such as an indoor channel, and a channel with low-speed Doppler spread, such as a pedestrian channel. The space diversity technique uses two or more transmission/reception antennas. In this technique, when a signal transmitted via one transmission antenna decreases in its signal power due to fading, a signal transmitted via the other transmission antenna is received. The space diversity can be classified into a reception antenna diversity technique using a reception antenna and a transmission diversity technique using a transmission antenna. However, because the reception antenna diversity technique is applied to a mobile station, it is difficult to install a plurality of antennas in the mobile station in view of the mobile station's size and its installation cost. Therefore, it is recommended that the transmission diversity technique should be used in which a plurality of transmission antennas are installed in a base station.
Particularly, in a 4th generation mobile communication system, a data rate of about 10 Mbps to 150 Mbps is expected, and an error rate requires a bit error rate (hereinafter referred to as “BER”) of 10−3 for voice, BER of 10−6 for data, and BER of 10−9 for image. The STTC is a combination of a multi-antenna technique and a channel coding technique, and is a technique bringing a drastic improvement of a data rate and reliability in a radio MIMO (Multi Input Multi Output) channel. The STTC obtains the receiver's space-time diversity gain by extending a space-time dimension of a transmitter's transmission signal. In addition, the STTC can obtain a coding gain without a supplemental bandwidth, contributing to an improvement in channel capacity.
Therefore, in the transmission diversity technique, the STTC is used. When the STTC is used, a coding gain having an effect of increasing transmission power is obtained together with a diversity gain which is equivalent to a reduction in a channel gain occurring due to a fading channel when the multiple transmission antennas are used. A method for transmitting a signal using the STTC is disclosed in Vahid Tarokh, N. Seshadri, and A. Calderbank, “Space Time Codes For High Data Rate Wireless Communication: Performance Criterion And Code Construction,” IEEE Trans. on Info. Theory, pp. 744-765, Vol. 44, No. 2, March 1998.
FIG. 1 is a block diagram schematically illustrating a general structure of a transmitter using STTC. Referring to FIG. 1, when P information data bits d1, d2, d3, . . . , dP are input to the transmitter, the input information data bits d1, d2, d3, . . . , dP are provided to a serial-to-parallel (S/P) converter 111. Here, the index P represents the number of information data bits to be transmitted by the transmitter for a unit transmission time, and the unit transmission time can become a symbol unit. The S/P converter 111 parallel-converts the information data bits d1, d2, d3, . . . , dP and provides its outputs to first to Pth encoders 121-1 to 121-P. That is, the S/P converter 111 provides a parallel-converted information data bit d1 to the first encoder 121-1, and in this manner, provides a parallel-converted information data bit dP to the Pth encoder 121-P. The first to Pth encoders 121-1 to 121-P each encode signals received from the S/P converter 111 in a predetermined encoding scheme, and then each provide their outputs to first to Mth modulators 131-1 to 131-M. Here, the index M represents the number of transmission antennas included in the transmitter, and the encoding scheme is an STTC encoding scheme. A detailed structure of the first to Pth encoders 121-1 to 121-P will be described later with reference to FIG. 2.
The first to Mth modulators 131-1 to 131-M each modulate signals received from the first to Pth encoders 121-1 to 121-P in a predetermined modulation scheme. The first to Mth modulators 131-1 to 131-M are similar to one another in operation except the signals applied thereto. Therefore, only the first modulator 131-1 will be described herein. The first modulator 131-1 adds up signals received from the first to Pth encoders 121-1 to 121-P, multiplies the addition result by a gain applied to a transmission antenna to which the first modulator 131-1 is connected, i.e., a first transmission antenna ANT#1, modulates the multiplication result in a predetermined modulation scheme, and provides the modulation result to a first multiplexer (MUX#1) 141-1. Here, the modulation scheme includes BPSK (Binary Phase Shift Keying), QPSK (Quadrature Phase Shift Keying), QAM (Quadrature Amplitude Modulation), PAM (Pulse Amplitude Modulation), and PSK (Phase Shift Keying). It will be assumed in FIG. 1 that since the number of encoders is P, 2P-ary QAM is used as a modulation scheme.
The first to Mth modulators 131-1 to 131-M provide their modulation symbols S1 to SM to first to Mth multiplexers 141-1 to 141-M, respectively. The first multiplexer 141-1 receives a modulation symbol S1 output from the first modulator 131-1, multiplexes a training sequence or a pilot sequence generated by a training sequence generator 151, and transmits its output via the first transmission antenna ANT#1. The training sequence generator 151 generates a sequence for channel estimation between a transmitter and a receiver, and generates 2 kinds of sequences: a sequence having a relatively long length; and a sequence having a relatively short length. The sequence having a relatively long length is a training sequence transmitted for initial channel estimation between the transmitter and the receiver, while the sequence having a relatively short length is a pilot sequence transmitted for channel estimation between the transmitter and the receiver during communication. During transmission of the training sequence and the pilot sequence, no information data is transmitted. Like the first multiplexer 141-1, other multiplexers, for example, the Mth multiplexer 141-M receives a modulation symbol SM output from the Mth modulator 131-M, multiplexes a training sequence or a pilot sequence generated by the training sequence generator 151, and transmits its output via the Mth transmission antenna ANT#M.
FIG. 2 is a block diagram illustrating a detailed structure of the first to Pth encoders 121-1 to 121-P of FIG. 1. For simplicity, a description will be made of only the first encoder 121-1. The information data bit d1 output from the S/P converter 111 is applied to the first encoder 121-1, and the first encoder 121-1 provides the information data bit d1 to tapped delay lines, i.e., delays (D) 211-1, 211-2, . . . , 211-(K−1). Here, the number of the delays, or the tapped delay lines, is smaller by 1 than a constraint length K of the first encoder 121-1. The delays 211-1, 211-2, . . . , 211-(K−1) each delay their input signals. That is, the delay 211-1 delays the information data bit d1 and provides its output to the delay 211-2, and the delay 211-2 delays an output signal of the delay 211-1. In addition, input signals provided to the delays 211-1, 211-2, . . . , 211-(K−1) are multiplied by predetermined gains, and then provided to modulo adders 221-1, . . . , 221-M, respectively. The number of the modulo adders is identical to the number of the transmission antennas. In FIG. 1, since the number of the transmission antennas is M, the number of the modulo adders is also M. Further, gains multiplied by the input signals of the delays 211-1, 211-2, . . . , 211-(K−1) are represented by gi,j,t, where i denotes an encoder index, j an antenna index and t a memory index. In FIG. 1, since the number of encoders is P and the number of antennas is M, the encoder index i increases from 1 to P, the antenna index increases from 1 to M, and the memory index K increases from 1 to the constraint length K. The modulo adders 221-1, . . . , 221-M each modulo-add signals obtained by multiplying the input signals of the corresponding delays 211-1, 211-2, . . . , 211-(K−1) by the gains. The STTC encoding scheme is also disclosed in Vahid Tarokh, N. Seshadri, and A. Calderbank, “Space Time Codes For High Data Rate Wireless Communication: Performance Criterion And Code Construction,” IEEE Trans. on Info. Theory, pp. 744-765, Vol. 44, No. 2, March 1998.
In order to decode the STTC-encoded signal transmitted by the transmitter, a receiver must have information on a channel characteristic that transmission signals transmitted via the plural transmission antennas experience while they are delivered to the receiver. In order to determine a channel characteristic of the transmission signals, the receiver performs a channel estimation process. Generally, in order to enable the receiver to perform channel estimation, a transmitter transmits a training sequence or a pilot sequence. Then, the receiver performs channel estimation by using the training sequence or the pilot sequence transmitted from the transmitter, and decodes a signal received according to the channel estimation result into a transmission signal transmitted by the transmitter.
In this manner, the transmitter transmits a training sequence or a pilot sequence for channel estimation, and during transmission of the training sequence or the pilot sequence, no information data is transmitted. The training sequence is periodically transmitted for synchronization between a transmitter and a receiver. Generally, when a channel environment does not undergo an abrupt change, channel estimation can be performed with only the training sequence. However, if a channel environment change speed is increased to the extent that a channel characteristic is changed within a relatively short time, for example, within one frame, the transmitter transmits a pilot sequence within a frame for the channel estimation. The receiver then accurately estimates the rapidly changing channel characteristic by detecting the pilot sequence, and correctly decode a received signal depending on the channel estimation result.
FIG. 3 schematically illustrates a frame format transmitted by the transmitter illustrated in FIG. 1. FIG. 3 will be described on the assumption that the number of transmission antennas included in the transmitter of FIG. 1 is 2. Referring to FIG. 3, each frame format transmitted through a first antenna ANT#1 and a second antenna ANT#2 is comprised of a training sequence transmission period (Training_Sequence) 311, information data transmission periods (Data) 313, 317 and 321, and pilot sequence transmission periods (Pilot) 315, 319, and 333. The training sequence transmission period 311 is a time period in which a training sequence for initial channel estimation between the transmitter and a receiver is transmitted. The information data transmission periods 313, 317, and 321 are time periods in which actual information data is transmitted, and the pilot sequence transmission periods 315, 319, and 333 are time periods in which a pilot sequence for channel estimation during transmission/reception of actual information data is transmitted. Herein, the time period in which the training sequence is transmitted is defined as “TT,” the time period in which the information data is transmitted is defined as “TD,” and the time period in which the pilot sequence is transmitted is defined as “TP.” Therefore, the first to Mth multiplexers 141-1 to 141-M of the transmitter (1) transmit a predetermined training sequence, i.e., a training sequence output from the training sequence generator 151, in the time period TT, (2) transmit information data, i.e., modulation symbols S1 to SM output from the first to Mth modulators 131-1 to 131-M, in the time period TD, and (3) transmit a pilot sequence, i.e., a pilot sequence output from the training sequence generator 151, in the time period TP.
FIG. 4 is a block diagram schematically illustrating a structure of an STTC transmitter having two encoders and 3 transmission antennas. Referring to FIG. 4, when 2 information data bits d1 and d2 are input to the transmitter, the input information data bits d1 and d2 are applied to an S/P converter 411. The S/P converter 411 parallel-converts the information data bits d1 and d2, and outputs the information data bit d1 to a first encoders 421-1 and the information data bit d2 to a second encoder 421-2. If it is assumed that the first encoder 421-1 has a constraint length K of 4 (constraint length K=4), an internal structure, illustrated in FIG. 2, of the first encoder 421-1 is comprised of 3 delays (1+2D+D3) and 3 modulo adders, wherein the number of delays and modulo adders is equal to a value smaller by 1 than the constant length K=4. Therefore, in the first encoder 421-1, the undelayed information data bit d1 applied to a first delay, a bit determined by multiplying a bit delayed once by the first delay by 2, and a bit delayed three times by a third delay are provided to a first modulo adder connected to a first modulator 431 of a first transmission antenna ANT#1. In this manner, outputs of the 3 modulo adders of the first encoder 421-1 are provided to the first modulator 431-1, a second modulator 431-2, and a third modulator 431-3, respectively. Similarly, the second encoder 421-2 encodes the information data bit d2 output from the S/P converter 411 in the same encoding method as that used by the first encoder 421-1, and then, provides its outputs to the first modulator 431-1, the second modulator 431-2, and the third modulator 431-3.
The first modulator 431-1 modulates the signals output from the first encoder 421-1 and the second encoder 421-2 in a predetermined modulation scheme, and provides its output to a first multiplexer 441-1. It is assumed herein that a modulation scheme applied to the transmitter is QPSK. Therefore, if an output signal of the first encoder 421-1 is b1 and an output signal of the second encoder 421-2 is b2, the first modulator 431-1 modulates the output signals in the QPSK modulation scheme, and outputs b1+b2*j, where j=√{square root over (−1)}. Like the first modulator 431-1, the second modulator 431-2 and the third modulator 431-3 modulate output signals of the first encoder 421-1 and the second encoder 421-2 in the QPSK modulation scheme, and then, provide their outputs to a second multiplexer 441-2 and a third multiplexer 441-3, respectively. The first to third multiplexers 441-1 to 441-3 multiplex output signals of the first to third modulators 431-1 to 431-3 with an output signal of a training sequence generator 451, and provide their outputs to first to third antennas ANT#1 to ANT#3, respectively. It will be assumed herein that a time TT in which the training sequence is transmitted is 10 (TT=10 ), a time TD in which the data information is transmitted is 10 (TD=10), and a time TP in which the pilot sequence is transmitted is 2 (TP=2). In this case, the first to third multiplexers 441-1 to 441-3 each transmit a training sequence output from the training sequence generator 451 for the first 10 symbols, transmit information data signals, i.e., modulation symbols S1 to S3 output from the first to third modulators 431-1 to 431-3, for the next 10 symbols, and transmit a pilot sequence output from the training sequence generator 451 for the next 2 symbols.
FIG. 5 is a block diagram schematically illustrating a receiver structure corresponding to the transmitter structure illustrated in FIG. 1. Referring to FIG. 5, a signal transmitted to the air by a transmitter is received through reception antennas of the receiver. It is assumed in FIG. 5 that there are provided N reception antennas. The N reception antennas each process signals received from the air. Specifically, a signal received through a first reception antenna ANT#1 is provided to a first demultiplexer (DEMUX) 511-1, and in the same manner, a signal received through an Nth reception antenna ANT#N is provided to an Nth demultiplexer 511-N. The first to Nth demultiplexers 511-1 to 511-N demultiplex signals received from the first to Nth reception antennas ANT#1 to ANT#N, and provide their outputs to a channel estimator 513 or a metric calculator 515. Here, the first to Nth demultiplexers 511-1 to 511-N demultiplex their input signals into information data, a training sequence, or a pilot sequence. In other words, the first to Nth demultiplexers 511-1 to 511-N demultiplex a received signal to be matched with a corresponding transmission period of the transmitter, as was described in conjunction with FIG. 3. That is, if the received signal corresponds to a period in which a training sequence is received, the first to Nth demultiplexers 511-1 to 511-N provide the received training sequence to the channel estimator 513. If the received signal corresponds to a period in which information data is received, the first to Nth demultiplexers 511-1 to 511-N provide the received information data to the metric calculator 515. If the received signal corresponds to a period in which a pilot sequence is received, the first to Nth demultiplexers 511-1 to 511-N provide the received pilot sequence to the channel estimator 513.
The channel estimator 513 channel-estimates signals output from the first to Nth demultiplexers 511-1 to 511-N by using a signal output from a training sequence generator 514, and outputs the channel estimation result to a hypothesis part 517. Here, the training sequence generator 514 generates a training sequence or pilot sequence generated in the transmitter, i.e., the same training sequence or pilot sequence as the training sequence or pilot sequence generated by the training sequence generator 151 as was described in conjunction with FIG. 1. Therefore, the channel estimator 513 performs initial channel estimation by comparing output signals of the first to Nth demultiplexers 511-1 to 511-N, received for the training sequence reception period, with a signal output from the training sequence generator 514. A method for performing initial channel estimation by using the training sequence is disclosed in A. F. Naguib, V. Tarokh, N. Seshadri, and A. Calderbank, “A Space Time Coding Modem For High Data Rate Wireless Communications,” IEEE Journal on selected areas in communications, pp. 1459-1478, Vol, No. 8. October 1998.
A possible sequence generator 519 generates all kinds of sequences which were possibly simultaneously encoded for information data bits transmitted by the transmitter, and provides the generated sequences to first to Pth encoders 521-1 to 521-P. Because the transmitter transmits information data by the P information bits, the possible sequence generator 519 generates possible sequences {tilde over (d)}1 . . . {tilde over (d)}P comprised of P bits. The P bits of the generated possible sequences are applied to the first to Pth encoders 521-1 to 521-P, and the first to Pth encoders 521-1 to 521-P encode their input bits in the STTC encoding scheme as was described in conjunction with FIG. 2, and then provide the encoded bits to first to Mth modulators 531-1 to 531-M. The first to Mth modulators 531-1 to 531-M each modulate the encoded bits output from the first to Pth encoders 521-1 to 521-P in a predetermined modulation scheme, and provide their outputs to the hypothesis part 517. The modulation scheme applied in the first to Mth modulators 531-1 to 531-M is set to any one of the BPSK, QPSK, QAM, PAM and PSK modulation schemes. Because a modulation scheme applied in the first to Mth modulators 141-1 to 141-M of FIG. 1 is 2P-ary QAM, the first to Mth modulators 531-1 to 531-M also modulate their input signals in the 2P-ary QAM modulation scheme.
The hypothesis part 517 receives modulation symbols output from the first to Mth modulators 531-1 to 531-M and the channel estimation value output {tilde over (S)}1 . . . {tilde over (S)}M from the channel estimator 513, generates a hypothetic channel output at a time when a sequence consisting of the signals output from the first to Mth modulators 531-1 to 531-M passed a channel corresponding to the channel estimation result, and provides the generated hypothetic channel output to the metric calculator 515. The metric calculator 515 receives the hypothetic channel output provided from the hypothesis part 517 and the signals output from the first to Nth demultiplexers 511-1 to 511-N, and calculates a distance between the hypothetic channel output and the output signals of the first to Nth demultiplexers 511-1 to 511-N. The metric calculator 515 uses Euclidean distance when calculating the distance.
In this manner, the metric calculator 515 calculates Euclidean distance for all possible sequences the transmitter can transmit, and then provides the calculated Euclidean distance to a minimum distance selector 523. The minimum distance selector 523 selects a Euclidean distance having the minimum distance from Euclidean distances output from the metric calculator 515, determines information bits corresponding to the selected Euclidean distance as information bits transmitted by the transmitter, and provides the determined information bits to a parallel-to-serial (P/S) converter 525. Although there are several possible algorithms used when the minimum distance selector 523 determines information bits corresponding to the Euclidean distance having the minimum distance, it is assumed herein that a Viterbi algorithm is used. A process of extracting information bits having the minimum distance by using the Viterbi algorithm is disclosed in Vahid Tarokh, N. Seshadri, and A. Calderbank, “Space Time Codes For High Data Rate Wireless Communication: Performance Criterion And Code Construction,” IEEE Trans. on Info. Theory, pp. 744-765, Vol. 44, No. 2, March 1998, so a detailed description thereof will not be provided for simplicity.
Because the minimum distance selector 523 determines information bits corresponding to the Euclidean distance having the minimum distance for all sequences generated from the possible sequence generator 519, it finally outputs P information bits of {circumflex over (d)}1,{circumflex over (d)}1, . . . ,{circumflex over (d)}P. The P/S converter 525 then serial-converts the P information bits output from the minimum distance selector 523, and outputs reception information data sequences {circumflex over (d)}1,{circumflex over (d)}1, . . . ,{circumflex over (d)}P.
As described above in conjunction with FIGS. 1 to 5, a transmitter using STTC transmits a training sequence and a pilot sequence for initial channel estimation and in-communication channel estimation, and during transmission of the training sequence and the pilot sequence, no information data is transmitted through all transmission antennas of the transmitter except the training sequence and the pilot sequence. Because no information data is transmitted during transmission of the training sequence and the pilot sequence, a data rate of the transmitter is decreased. For example, when the transmitter has 2 transmission antennas, a training sequence and a pilot sequence are transmitted through both of the 2 transmission antennas in a period where the training sequence and the pilot sequence are transmitted. Therefore, in the period where the training sequence and the pilot sequence are transmitted, it is impossible to transmit information data. Due to the impossibility of transmitting information data, a data rate of the transmitter is decreased, and if there are a total of L pilot sequence transmission periods and information data transmission periods for one frame, the entire overhead becomes (LTP+TT)/(LTP+LTD+TT). For example, assuming a period TD in which the information data is transmitted has a length 3 times longer than a period TP in which the pilot sequence is transmitted, if the L is set to a relatively large value, an overhead of the transmitter is 25% of the entire overhead. That is, a decrease in a data rate of the transmitter results in a reduction in the system performance.